Thermo-optic liquid-crystal device for real-time display of animated images

ABSTRACT

The invention provides a liquid crystal imaging system which employs a transitory thermo-optic effect to permit notably the reproduction of T.V. images. This transitory effect is characterized by the fleeting appearance of scattering cybotactic zones in the transparent liquid phase of a mesomorphic material, at the very beginning of the transition to a nematic transparent ordered phase. The duration of the scattering period can be considerably shortened by subjecting the mesomorphic material layer to the action of an alternating electrical field.

FIELD OF THE INVENTION

The invention relates to a liquid crystal imaging system, and morespecifically, to an imaging system which employs a transitorythermo-optic effect to permit notably the reproduction of animatedimages transmitted at a distance, such as for example televised images.

BACKGROUND OF THE INVENTION

In the prior art, devices are known which employ thin layers ofmesomorphic materials subjected to a thermo-optical effect to reproduceimages transmitted at a distance. In a general manner, these devicescomprise a cell in which the material, interposed between twotransparent sheets, is preheated to a temperature which is lower byseveral degrees Centigrade than the transition temperature from themesomorphic phase utilised (the smectic or cholesteric phase) either toa further mesomorphic phase or to an isotropic liquid phase. The mode ofprocedure is such that the layer is uniformly oriented, either due toprevious processing of the face of the support sheets contacting thelayer, or due to application of an alternating field; thus, the layer isperfectly and uniformly transparent.

A beam of light, generally selected within the near-infrared zone andintensity modulated by a signal transmitting the image to be recorded,sweeps the cell. When the energy supplied at one point of the cell bythe beam has been adequate to locally heat the liquid crystal to beyondthe transition point at which the latter is converted to an isotropicliquid, there is locally formed, during the rapid return to the initialmesomorphic phase, a disordered texture (known as a focal-conicstructure) which is optically scattering; on the contrary, when theluminous energy supplied by the modulated beam has been inadequate, thecorresponding point retains the initial, uniform orientation and remainstransparent. On projecting the image of the cell on to a screen by anappropriate optical device, the non-scattering spots of the cell appearas brilliant spots and the diffusing spots as dark spots.

The focal-conic structure thus set up within the material in themesomorphic phase is stable. The devices constituted in this manner arestorage devices able to preserve the recorded image for a durationvarying between several hours and several days. In order to erase therecorded image, it is possible either to scan the cell with the luminousbeam at full intensity and to then slowly recool the layer, or to submitthe layer to the action of an alternating field, with or withoutsweeping by the beam.

The erasive time is always relatively slow (0.1 s for the fastestdevice). Consequently, it is impossible to employ such systems toreproduce animated images, for example televised images, since it isknown that, in order to avoid scintillation, the projection rate must beat least equal to some 20 images per second.

SUMMARY OF THE INVENTION

In order to palliate this drawback of the prior art, the inventionproposes to utilise a new thermo-optic effect, observed in materialsexhibiting a nematic state isotropic phase transition. This effect ischaracterised by the fleeting appearance of scattering cybotactic zonesin the body of the transparent liquid phase, at the very beginning ofthe transition to the transparent nematic phase. Since this effect isessentially transitory, the recorded image spontaneously vanishes, andthis takes place in a time sufficiently short to permit the achievementof a recording rate of 25 images per second.

BRIEF DESCRIPTION OF THE DRAWING

This invention, together with is features, advantages and objects willbe better understood from the following detailed description when readin conjunction with the attached drawing, in which:

FIG. 1 shows a first embodiment of the device for the reproduction ofimages according to the invention, wherein the thin layer is locallyheated by a moving beam of light.

FIG. 2, which relates to a second embodiment of the invention, shows thearrangement on one of the support sheets of the cell of a conductornetwork feeding a matrix of resistive elements permitting the localheating of the thin layer;

FIG. 3 shows a sectional view of the cell of which the element of FIG. 2forms a part;

FIG. 4 shows, in accordance with the invention, a device for projectionon a screen utilising the said device for the reproduction of images.

DETAILED DESCRIPTION

Research work effected in the laboratory of the Applicants and relatingto the isotropic liquid mesomorphic phase transition have shown that,notably in those materials wherein thus mesomorphic phase is a nematicphase, there is observed an extremely fleeting pre-transitional effectcharacterised by the appearance of cybotactic zones within the isotropeliquid phase. These zones are constituted of ordered, birefringentmicro-domains, without an orientation relationship between each otherand constituting the germs of the mesomorphic phase which will beprogressively established within the liquid. If the walls contacting thematerial have been subjected to surface treatment promoting uniformorientation of the mesomorphic phase, the germs are spontaneouslyreoriented relative to each other and afford a mesomorphic layer theorientation of which is uniform and which is thus as perfectlytransparent as is the original isotropic liquid phase. Consequently, ifa layer of a material exhibiting for example an isotropic liquid-nematictransition is arranged between two sheets of glass which have beensubjected to an appropriate surface treatment (coating with silica inthe case of a material having positive dielectrical anisotropy, and withsilane in the contrary case for instance), and cooled from isotropicliquid state to the nematic state, the crystallographic evolutionpreviously described will be accompanied by the following evolution ofthe optical properties: the layer remains transparent until thetransition temperature is reached, and then becomes temporarily stronglydiffusing when the cybotactic appear, becoming once again perfectlytransparent when the mesomorphic phase is established. The measurementseffected have shown that the lapse of time during which the layerbecomes scattering is extremely short and can not exceed 0.05 s in thecase of a nematic material with negative dielectric anisotropy. Theyhave also shown that the duration of the scattering period could bestill further considerably shortened by subjecting the layer to theaction of an alternating electrical field of some kilocycles per secondfrequency, thereby accelerating the realignment of the molecules in thenematic phase.

The present invention proposes to profit from the observations describedhereinabove to effect image reproduction without memory effect, andnotably to reproduce animated scenes in "real time". For this purpose, aliquid crystal cell is prepared with a material exhibiting thepre-transitional effect previously described and disposed in a thin,uniformly oriented layer. The layer is maintained at a temperature suchthat the material is in the mesomorphic phase. Selective heating means,triggered by the video signal transmitting the image to be recorded,make it possible to increase up to that temperature at which thematerial is in an isotropic liquid phase those of the points of thelayer which it is desired to render diffusing; these points willcorrespond, depending on the modes of lighting and observation of thecell, either to dark or luminous points of the image to be recorded; onreturning from the isotropic liquid state to the oriented mesomorphicstate, they diffuse the light during the short period of time in whichthe cybotactic zones are formed; the non-heated spots remaintransparent. Thus, with such a cell it becomes possible to reproduceanimated images by a process highly comparable to that of thefluorescent screen of a television cathode tube, wherein the spotsexcited by the beam of electrons emit a remanent light duringapproximately the duration of an image. The essential difference isthat, in the present invention, the "excited" spots emit no light butpermit modulation of the light derived from an external source, whichmay thus be selected to be extremely intense, in such manner for exampleas to project the images recorded on the cell on to a large-dimensionscreen.

FIG. 1 shows a first embodiment of the device for the reproduction ofimages according to the present invention, in which embodiment the meansfor selective heating of the cell are constituted by a luminous beam.

A liquid crystal cell 1 is constituted by a thin layer 10, approximately10 to 20 μm thick, of a material exhibiting an isotropic liquid/nematicphase transition; the said material may for example be M.B.B.A.(paramethoxybenzylidene-butyl aniline) or E.B.B.A(paraethoxybenzylidene-butyl aniline) or a mixture of these twosubstances. The thin layer 10 is disposed between two parallel glasssheets 11 and 12, both coated on their inner face with transparentelectrodes 13 and 14 constituted by a deposit of tin oxide or indium, ora mixture of these two oxides. Silane coatings 15 and 16 disposed at theinterface of the thin layer 10 and of the electrodes 13 and 14 promoteuniform orientation of the material in the nematic phase. A voltagegenerator 2 permits application between the two elecrodes 13 and 14 ofan alternating voltage of approximately 30 volts and several frequencykilocycles. This generator also permits the application of aunidirectional voltage between two ends of the electrode 14; thus, thetwo electrodes act as heating resistors and permit maintaining the thinlayer 10 at a temperature very close to the nematic phase/isotropicliquid transition temperature, such that the material is in the nematicphase.

The luminous ray emitted by a source 3 is collected by the condenser 30,modulated by the modulator 31, deflected by the deflector 32, andfocused by the objective 33 which thus supplies a moving beam 300converging in the plane of the layer 10. The source 3 must possesssufficient intensity, taking into account the sweep velocity, to carrythe volume of the layer 10 surrounding the point of convergence of thebeam 300 from the mesomorphous phase to the isotropic liquid phase; itmay advantageously be constituted by a YAG laser continuously emitting abeam of 1.06 μm wavelength. The modulator 30 may be an electro-opticalmodulator; it permits interruption of the luminous beam intensity whenthe swept point is required to remain in the mesomorphous phase. Thedeflector 32 permits the beam 300 to sweep the entire surface of thecell 1; it may be a mechanical deflector or, if the sweeping frequenciesare high, as in the case of sweeping at television cadence, an assemblyof two electro-optical or acousto-optical deflectors. The modulator 31and the deflector 32 are triggered by a control generator 4 receivingthe video signal characteristic of the images to be reproduced on thecell.

The energy carried by the luminous beam 300 is absorbed by the thinlayer 10, if the material constituting the latter is sufficientlyabsorbent, either on itself or by means of impurities incorporated forthis purpose, relative to the radiation emitted by the source 3. In theprecise case of the device shown in FIG. 1, wherein the materialutilised absorbs radiation emitted by the YAG source, withinnear-infrared range, only to very slight degree, the conducting layers13 and 14 fulfill a triple role; on the one hand, they absorb nearly allthe energy of the beam 300 and communicate the heat given off to thepart of the layer 10 which they enclose; on the other hand, they make itpossible to subject this same layer to an alternating electrical fieldwhich, as already previously explained, reduces the time of appearanceof scattering phenomena; finally, as stated hereinabove, they functionas a heating resistance to maintain the thin layer at adequatetemperature. These points swept by the beam 300 which have been broughtto the isotropic liquid state, appear transitorially as scattering onsubsequent cooling.

FIGS. 2 and 3 relate to a second embodiment of the device for thereproduction of images according to the invention, wherein the means forselectively heating the cell are constituted by a matrix of resistiveelements incorporated in the cell itself and fed by a mutiplexingdevice.

In this embodiment, as in the preceding one, a liquid crystal cell isconstituted by interposing between two glass support sheets a thin layerof a material exhibiting a mesomorphic phase (for example a nematicphase), one of the sheets comprising the same coatings as in thepreceding case and the other supporting the matrix of resistantelements. FIG. 2 shows a plan view of the latter support sheet, and FIG.3 shows a section through the cell containing the support sheet of FIG.2, section being effected along the straight line marked CC in FIG. 2.

Referring to both these figures, it will be appreciated that there aredeposited on the support sheet 11, in the form of a regular matrix,resistive elements in the form of bars, such as the element 130; theseelements are transparent and are constituted by a deposit of indium ortin oxide, or a mixture of these two oxides. The current inputs andoutputs of these resistive elements are constituted by linear conductorelements, comprising a thin metal film for example of gold, covering theends of the resistive elements; an analogous column of resistiveelements is connected in parallel to a single input conductor element,such as the linear conductor element 131; similarly, all the resistiveelements of one and the same line are connected in parallel to the sameoutput conductor element, such as the element 132. The column conductorelements, such as the line conductor elements, are thus connected inparallel with each other and are uniformly spaced, the first and thesecond assembly being orthogonal. Terminals such as 133 and 134,uniformly distributed over the periphery of the support plate, permitready connection between these elements and a device for feeding bymultiplexing. Insulating deposits, such as 135, are interposed betweenthe linear conductor elements at the point of intersection thereof; theyare constituted for example by a thin layer of SiO or of a photoresist.

Referring to FIG. 3, this figure also shows that the assembly of theelements 130, 131, 132 and 135, to the exclusion of the terminals 133and 134, are coated with a thin layer of silane 15 promoting uniformorientation of the mesomorphic phase. This same FIG. 3 also shows thethin layer 10 of mesomorphic material and also the second support sheet12, successively coated with a transparent or reflecting electrode 14and a thin deposit of silane 16.

The various elements coating the inner face of the support sheet 11 aredeposited by successive vaporisation in vacuo, through appropriatemasks.

Feeding of the resistive elements of the matrix is effected inaccordance with the known multiplexing process, which may be summed-upin the following manner:

A triggering generator operating by multiplexing receives the videosignal containing the information relating to the images to be recorded.The signals corresponding to the successive points of one and the sameline are stored; when all the information concerning a line has beenreceived, the device connects, via the channel of the adequate lineterminal, the linear conductor of the same rank or order as the linereceived with all the terminals corresponding to those points of theline which require to be excited, in accordance with the stored signals.The voltage is thus applied between the terminals of only thoseresistive elements of one and the same line which have been selected bythe channel of the column terminals. Whilst the resistive elements ofthe line of rank or order N are put under voltage, the generator storesthe information concerning the line of order N + 1; as soon as thisinformation is totally received, the line of order N is disconnected andreplaced by the line of order N + 1, and the same process recommencesonce again. Those of the resistive elements which are thus put undervoltage communicate the heat received to the subjacent element of thelayer 10, which then passes from the mesomorphic phase to the isotropicliquid phase, and becomes temporarily scattering on subsequent cooling.

In addition to the multiplexing pulses, the control or triggeringgenerator applies a constant peak value alternating voltage between onthe one hand the assembly of line terminals and columns disposed on thesupport sheet 11 and on the other hand the electrode 14 disposed on thesupport sheet 12. As previously states, this voltage diminishes theduration during which the points excited in the layer 10 remaindiffusing.

Depending on the transparent or reflecting nature of the electrode 14,the cell described hereinabove may be utilised by transmission or byreflection.

FIG. 4 shows an example of utilisation of the device for thereproduction of images according to the invention, for the projection ona screen of images reproduced by the liquid crystal cell. It concerns,more specially, the case of the cell described by FIGS. 2 and 3, but itmay readily be applied to the case illustrated by FIG. 1.

FIG. 4 shows the liquid crystal cell 100, fed by the triggeringgenerator 7 operating by multiplexing, which receives the video signaland distributes the feed voltages to the matrix of resistive elements. Asource of light 5 illuminates the entire surface of the cell withparallel light by means of the condenser 50. The light transmitted bythe cell is collected by the objective 51 which conjugates the plane ofthe thin layer 10 of mesomorphic material with the plane of a screen 52.A diaphragm 53, pierced with a circular orifice 530, arranged at thefocus of the objective 51, permits passage to the screen 52 of onlythose beams which emerge from the cell 100 under quasi-normal emergence.Under these conditions, the points diffusing in the layer 10 areprojected as dark points on the screen 52.

To adapt this projection device to the device shown in FIG. 1, itsuffices to interpose on the path of the beam 300 in FIG. 1, asemi-transparent or dichroic mirror arranged at 45° relative to theoptical axis of the system and reflecting onto the cell 10 the beam ofparallel light emanating from the objective 50 of FIG. 4; the opticalaxis of the latter is then arranged parallel to the plane of the liquidcrystal cell and at 45° relative to the semi-transparent or dichroicmirror.

What we claim is:
 1. A method of utilizing a thermo-optical effect forreal-time display of successive images in the form of scattering pointsin a thin transparent layer of a material exhibiting a mesomorphic statewith direct transition to an isotropic liquid state comprising the stepsof:maintaining said layer in uniform orientation at a first temperaturein said mesomorphic state; temporarily heating said points to a secondtemperature in said isotropic liquid state; subsequently cooling fromsaid isotropic liquid state to said mesomorphic state in uniformorientation so that light scattering cybotactic zones are transitorilyformed in said points; and applying an AC voltage during the transitionfrom said isotropic liquid state to said mesomorphic state to reduce theduration of the cybotactic zones and to maintain said layer transparentin said mesomorphic state.
 2. A method according to claim 1, in whichsaid mesomorphic state is a nematic state.